doc: Create usage guide section

Start structuring our docs a little better starting with a usage guide.
There are a number of changes happening here. We're going to start with
the installation guide, which is simply moved to the appropriate
subdirectory.

Signed-off-by: Stephen Finucane <stephen@that.guru>

doc/contents.rst

@@ -7,6 +7,8 @@ Sphinx documentation contents
 .. toctree::
    :maxdepth: 2
 
+   usage/installation
+
    intro
    tutorial
    man/index

doc/usage/installation.rst

@@ -1,5 +1,3 @@
-:orphan:
-
 =================
 Installing Sphinx
 =================